Issue 35

S. Lesz et alii, Frattura ed Integrità Strutturale, 35 (2016) 206-212; DOI: 10.3221/IGF-ESIS.35.24 207 I NTRODUCTION etallic glasses (MGs) are poised to be mainstay materials for the 21 st century due to the unique physical and chemical properties, which offers a great potential for application in industry, medicine, energy systems, microelectronics, aeronautics and many other fields. The first reported scientifically obtained metallic glass (MG) was the alloy Au 75 Si 25 produced at Caltech by Klement, Willens & Duwez in 1960, by extremely rapid cooling of the melted alloy [1]. In the 1960s, Chen and Turnbull developed amorphous alloys of Pd-Si-Ag, Pd-Si-Cu, and Pd-Si-Au [2]. Chen also fabricated an amorphous Pd-Cu-Si alloy with a diameter of up to 1 mm that could be considered to be a bulk metallic glass (BMG) [3]. A study of the mechanical properties of these novel materials was first reported in 1971 by Masumoto and Maddin [4]. In recent years a great expansion in the number of alloy compositions known to give bulk metallic glasses (BMGs) have occurred. The first Fe-based bulk metallic glasses (BMGs) were prepared in 1995 [5]. Since then, Fe-based bulk metallic glasses have been studied as a novel class of engineering materials, which have a good glass forming ability and soft magnetic properties [6,7]. For example, in 2004, Inoue et al. synthesized [(Fe x Co 1−x ) 0.75 B 0.2 Si 0.05 ] 96 Nb 4 (x = 0.1 and 0.5 at.%). BMGs exhibit good soft magnetic properties, as well as super-high fracture strength of 3000–4000MPa and ductile strain of 0.002 [6]. Bulk metallic glasses (BMGs) possess superior mechanical properties such as high strength and great elastic strain making them ideal candidates for structural applications. However, the poor ductility and brittle fracture exhibited in nearly all monolithic BMGs limit their structural application. Hence, a well understanding fracture morphology and mechanical properties is important for designing performance of BMGs. The purpose of the paper was an investigation of the mechanical properties, structure and particularly fracture morphology of the Fe 36 Co 36 B 19 Si 5 Nb 4 bulk metallic glass (BMG) after compression. E XPERIMENTAL PROCEDURE he master alloy ingots with compositions of Fe 36 Co 36 B 19 Si 5 Nb 4 were prepared from the pure Fe, Co, Nb metals and non-metallic elements: Si, B, in an argon atmosphere. The alloy composition represents nominal atomic percentages. The investigated material was cast in form of rods with diameter of  =2, 3 and 4 mm. According to Johnson, cooling rate achieved for an as-cast diameter R can be estimated as: T =10/ R 2 (cm) [8]. Thus, the achieved cooling rate in the rod-shaped samples with  =2, 3 and 4 mm in diameter could be estimated to be 1000, ~444 and 250 K/s. Obviously the smaller the as-cast diameter, the larger the cooling rate is achieved. The rods were prepared by the pressure die casting. The following experimental techniques were used: X-ray diffraction (XRD) phase analysis method to test the structure, scanning electron microscopy (SEM) to investigate fracture morphologies obtained after decohesion process in compression test. The XRD method has been performed by the use of diffractometer XRD 7, Seifert-FPM, with filtered Co-Kα radiation. The morphology of fracture surfaces after decohesion process in compression test was examined by means of the scanning electron microscope (SEM) SUPRA 25, ZEISS. The measurement of mechanical properties, like: Young modulus - E , compressive stress - σ c , elastic strain – ε, unitary elastic strain energy – U u , were made in compression test. Compression tests for bulk metallic glasses were performed on ZWICK 100 testing machine at a strain rate of 5 × 10 -4 s -1 , at room temperature. For each group, five specimens were tested, and averaged data were used. R ESULTS AND DISCUSSION t was found from the obtained results of structural studies performed by X-ray diffraction that diffraction pattern of surface rods with  =2, 3 and 4 mm in diameter of Fe 36 Co 36 B 19 Si 5 Nb 4 alloy consists of a broad diffused halo typical for the amorphous phase (Fig. 1). The mechanical properties of samples, including Young’s modulus - E , compressive stress – σ c and elastic strain - ε, unitary elastic strain energy – U u , are listed in Tab. 1 and Fig. 2. As shown in the Fig. 2, the Fe 36 Co 36 B 19 Si 5 Nb 4 BMG exhibits elastic strain – ε of 0.75 to 0.94%. Young’s modulus - E and compressive stress - σ c , unitary elastic strain energy – U u of the glassy alloy rods are in the range of 105-191 GPa, 790 – 1794 MPa, 17-25 kJ/m 2 , respectively. The values of unitary elastic strain energy – U u decrease with the increasing diameter T I